There are a large number of different types of jet engines, all of which achieve forward thrust from the principle of jet propulsion. Gas turbines are rotary engines that extract energy from a flow of combustion gas. They have an upstream compressor coupled to a downstream turbine with a combustion chamber in-between. In aircraft engines, those three core components are often called the “gas generator.” There are many different variations of gas turbines, but they all use a gas generator system of some type. A turbojet engine is a gas turbine engine that works by compressing air with an inlet and a compressor (axial, centrifugal, or both), mixing fuel with the compressed air, burning the mixture in the combustor, and then passing the hot, high pressure air through a turbine and a nozzle. The compressor is powered by the turbine, which extracts energy from the expanding gas passing through it. The engine converts internal energy in the fuel to kinetic energy in the exhaust, producing thrust. All the air ingested by the inlet is passed through the compressor, combustor, and turbine.
Although these and other type jet engines work well for their stated design goals, their respective design configurations do not scale well to smaller and perhaps unmanned situations. For example, compressors and associated jet fuel tanks cannot be efficiently scaled to smaller vehicles. As such, although jet engine technology has enjoyed enormous success for larger aircraft applications, it has not been applied effectively to smaller vehicles.